Method for producing long obliquely stretched film

ABSTRACT

A long obliquely stretched film is obtained having an orientation axis in a direction with an angle θs of 10° or more and 85° or less from a width direction by grasping both side ends in the width direction of a longitudinally stretched film with an average retardation Re 1  by grasping means; stretching the film under a condition that a long obliquely stretched film sample with an average retardation Re 2  (Re 2  is smaller than Re 1 ) can be obtained and traveling speeds of the grasping means are substantially equal at the film both side ends through a preheating zone, a stretching zone and a fixing zone; releasing the both side ends of the film from the grasping means; and taking it up around a winding core.

TECHNICAL FIELD

The present invention relates to a method for producing a long obliquelystretched film, a method for producing a retarder, and a liquid crystaldisplay device comprising the retarder.

BACKGROUND ART

In a liquid crystal display device, various retarders are used foroptical compensation such as prevention of coloring and expansion ofview angles. Retarders used in the liquid crystal display device are ina rectangular shape in general and have an orientation axis in adirection inclined with respect to a side thereof. In a liquid crystaldisplay devices, such a retarder is laminated on a polarization platesuch that the orientation axis of the retarder and a polarizationtransmission axis of the polarization plate get into a desired angle.

As a method of manufacturing a retarder having an orientation axis in adirection inclined with respect to its side as above, such a method iswidely known that a transparent resin film (raw material film) isstretched longitudinally or laterally to be oriented so as to obtain along stretched film and then the stretched film is cut in a rectangularshape with a predetermined angle with respect to the side of thestretched film. However, with this method, even if cutting is done sothat the maximum area can be obtained, a cutting loss is generated allthe time to lower use efficiency of the stretched film, which is aproblem. On the other hand, a long stretched film oriented obliquelywith a predetermined angle can be cut away in parallel with the side toincrease use efficiency of the stretched film.

A method of obtaining a film with a obliquely oriented orientation axisby stretching has been known. For example, in Patent Document 1, amethod for producing a obliquely stretched film which is stretched in adirection oblique to a film longitudinal direction is disclosed in whichboth side ends of a film is grasped between two rows of chucks runningon tenter rails arranged so that traveling distances of the chucks in apredetermined traveling section are different and is made to travel.

Patent Document 2 discloses a method of stretching an optical polymerfilm characterized in that, in a method of stretching an optical polymerfilm stretched by giving a tension while holding both side ends of acontinuously supplied polymer film by holding means and advancing theholding means in a longitudinal direction of the film, stretching isperformed while a locus L1 of the holding means from a substantial holdstart point to a substantial hold release point at one side end of thepolymer film and a locus L2 of the holding means from the substantialhold start point to the substantial hold release point at another sideend of the polymer film, and a distance W between the two substantialhold release points satisfy a relation of |L2−L1|>0.4 W, bearingproperties of the polymer film are maintained and a state where avolatile component content is 5% or more is present and then, thevolatile component content is lowered while the film is made to shrink.

Patent Document 3 discloses a method for producing a long optical filmhaving an optical axis (orientation axis) being in a direction neitherin parallel nor perpendicular to a flow direction of the long filmobtained by stretching a long film made of a thermoplastic resin, inwhich the method for producing a long optical film characterized inthat, in a region where the film is substantially stretched, stretchingis performed so that magnitude of moving speeds at both opposing sideends in the width direction of the films are equal and the travelingdistances are different, and at least one of a pair of jigs holding thewidth-direction both side ends of the film is moved on a rail in a wavedshape with respect to the film surface. Moreover, Patent Document 3describes that the stretching process may be repeated several times orthe stretching process may be performed after stretching in alongitudinal direction or a lateral direction in advance.

However, with these obliquely stretching methods, wrinkles or twists caneasily occur obliquely. Thus, it is substantially impossible to obtain awide film having a uniform thickness in the width direction and anorientation axis uniformly oriented in an oblique direction at 10° ormore and 85° or less. Thus, it has not been possible to mass-produce along and wide optical film industrially in which an orientation axis isoriented obliquely (in a direction largely offset from the film widthdirection or longitudinal direction).

-   Patent Document 1: Japanese Patent Laid-Open No. 2-113920-   Patent Document 2: Japanese Patent Laid-Open No. 2002-86554    (corresponding to U.S. Pat. No. 6,746,633)-   Patent Document 3: Japanese Patent Laid-Open No. 2003-232928

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a method for producinga wide and long stretched film or retarder film having a uniformthickness in the width direction and an orientation axis uniformlyoriented in an oblique direction at 10° or more and 85° or less, aretarder film obtained by the method, and a liquid crystal displaydevice comprising the retarder film.

Means for Solving the Problems

The inventors have earnestly proceeded with studies in order to achievethe above object and found that in two-stage stretching which obliquelystretching is performed after longitudinal stretching, by controlling arelation of average retardations generated in the longitudinalstretching and obliquely stretching, respectively, and a stretchingangle of the obliquely stretching, or specifically by stretching a filmwith an average retardation Re₁ obtained by longitudinally stretching ina direction with an angle θe oblique to the width direction under acondition that a long obliquely stretched film sample with the averageretardation Re₂ (Re₂ is smaller than Re₁) is obtained from a rawmaterial film with substantially zero average retardation, a obliquelystretched film with a uniform width-direction thickness having anorientation axis in a direction at 10° or more and 85° or less from thewidth direction and in a direction with an angle larger than the aboveee and no wrinkles or twists can be obtained. And the present inventionhas been completed based on this finding.

According to the present invention,

(1) a method for producing a long obliquely stretched film having anorientation axis in a direction with an angle θs of 10° or more and 85°or less from the width direction, the method comprising:

a) a step of longitudinally stretching a long raw material film toobtain a longitudinally stretched film with an average retardation Re₁;and a step of stretching the longitudinally stretched film in adirection with an angle θe oblique to the width direction, wherein

b) the step of stretching in the direction with the angle θe isperformed under a condition that a long obliquely stretched film samplewith the average retardation Re₂ (Re₂ is smaller than Re₁) is obtainedfrom the long raw material film;

(2) the method for producing a long obliquely stretched film accordingto the above (1), in which a stretching temperature of the stretchingstep in the direction of the angle θe is higher than a stretchingtemperature of the step of longitudinal stretching; and(3) the method for producing a long obliquely stretched film accordingto the above (1) or (2), in which a stretching magnification of thestretching step in the direction at the angle θe is smaller than astretching magnification of the longitudinal stretching step areprovided.

Also, according to the present invention,

(4) a method for producing a long obliquely stretched film having anorientation axis in a direction with an angle θs of 10° or more and 85°or less from the width direction, the method comprising:

a) a step of winding a longitudinally stretched film off a winding bodyof the longitudinally stretched film with the average retardation Re₁obtained by longitudinally stretching a long raw material film; a stepof grasping both side ends in the width direction of the longitudinallystretched film by grasping means; a step of stretching thelongitudinally stretched film in the direction of the angle θe obliqueto the width direction through a preheating zone, a stretching zone anda fixing zone to obtain a obliquely stretched film; a step of releasingthe both side ends of the obliquely stretched film from the graspingmeans; and a step of taking up the obliquely stretched film around awinding core, wherein

b) the step of stretching in the direction of the angle θe is performedunder a condition that a long obliquely stretched film sample with theaverage retardation Re₂ (Re₂ is smaller than Re₁) is obtained from thelong raw material film; and

c) traveling speeds of the grasping means are substantially equal atboth side ends of the film is provided.

As a preferred mode of the present invention,

(5) a method for producing a long obliquely stretched film according tothe above (4), in which a film running direction in the fixing zone isinclined with an angle of θ1 from the film running direction in thepreheating zone and the angle θ1 is not more than θe is provided.

Moreover, according to the present invention,

(6) a long obliquely stretched film obtained by the method according toany one of the above (1) to (5);

(7) a long laminate film obtained by laminating the long obliquelystretched film according to the above (6) and a long polarizer withtheir longitudinal directions aligned;

(8) a method for producing a retarder comprising a step of trimming thelong obliquely stretched film according to the above (6) to apredetermined size along a direction perpendicular to or parallel withits longitudinal direction;

(9) a method for producing a polarizing plate including a step oftrimming the long laminate film according to the above (7) to apredetermined size;

(10) a liquid crystal display device comprising the retarder obtained bythe method according to the above (8).

(11) a liquid crystal display device comprising a polarizing plateobtained by the method according to the above (9); and

(12) a liquid crystal display device according to the above (10) or (11)comprising a reflective-type liquid crystal panel are provided.

Advantages of the Invention

According to a producing method of the present invention, a wide andlong obliquely stretched film can be easily obtained in which athickness in the width direction is uniform and an orientation axis isuniformly oriented in a direction at 10° or more and 85° or less,preferably 12° or more and 85° or less, more preferably 40° or more and85° or less, or particularly preferably 51° or more and 85° or less withrespect to the width direction. The long stretched film with theobliquely oriented orientation axis is suitable as a retarder of aliquid crystal display device and the like. Specifically, when laminatedwith other long optical elements used for a liquid crystal displaydevice such as a polarizing plate with an orientation axis inclined at agiven angle, by using the stretched film having an orientation axisoblique to the longitudinal direction, lamination by roll-to-roll withother long optical elements can be realized. Since a film having anorientation axis in parallel with the width direction (TD direction) orlongitudinal direction (MD direction) of a long film needs diagonaltrimming, that leaves a large waste portion. Since the stretched filmhaving an orientation axis oblique to the MD direction obtained by theproducing method in the present invention may be trimmed in parallelwith the MD direction or TD direction, that leaves little film wasteportion, which is excellent in productivity. Also, the retarder obtainedby the producing method in the present invention has a wider view angleof its display screen when used in a liquid crystal display device,particularly in a reflective-type liquid crystal display device, and canresult in preventing drop in contrast or coloring of the display screen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a conceptual diagram illustrating an example of a tenterstretching machine to which a producing method in the present inventioncan be suitably applied.

FIG. 2 a diagram illustrating grasping means of a rail portion in thestretching machine in FIG. 1.

FIG. 3 a diagram for explaining rail arrangement in the stretchingmachine in FIG. 1.

EXPLANATION OF SYMBOLS  1: film 10: temperature-controlled room 11: rail12: grasping means 13: boundary between preheating zone and stretchingzone 14: boundary between stretching zone and fixing zone 21: wind-offroll 22: wind-up roll 47: running direction of film in preheating zone49: running direction of film in fixing zone S1, S2: stretching startpoint E1, E2: stretching end point

BEST MODE FOR CARRYING OUT THE INVENTION

A method for producing a long obliquely stretched film having anorientation axis in a direction with an angle θs of 10° or more and 85°or less from the width direction according to the present inventioncomprises a) a step of longitudinally stretching a long raw materialfilm to obtain a longitudinally stretched film with an averageretardation Re₁; and a step of stretching the longitudinally stretchedfilm in a direction with an angle θe oblique to the width direction; b)in which the step of stretching in the direction with the angle θe isperformed under a condition that a long obliquely stretched film samplewith the average retardation Re₂ (Re₂ is smaller than Re₁) is obtainedfrom the long raw material film.

The long raw material film used in the present invention is a long filmmade of a transparent resin. The term long means those having a lengthof at least approximately 5 times or more of a width of a film or alaminate body, and preferably having a length of 10 times or more; andmore specifically having a length being such an extent that is wound ina roll state for storage or transport. The transparent resin is a resinwhich is transparent to a desired wavelength. The transparent resin ispreferably a thermoplastic resin. Also, the transparent resin ispreferably a resin with a positive specific birefringence value. Thetransparent resins include polycarbonate resin, polyethersulphone resin,polyethylene terephthalate resin, polyimide resin,polymethylmethacrylate resin, polysulphone resin, polyarylate resin,polyethylene resin, polyvinylchloride resin, diacetylcellulose,triacetylcellulose, polystyrene resin, polyacryl resin, olefin polymerhaving alicyclic structure (alicyclic olefin polymer) and the like.Among them, alicyclic olefin polymer is preferable.

The olefin polymers having alicyclic structure include norbornene resin,single-ring cyclic olefin resin, cyclic conjugated diene resin, vinylalicyclic hydrocarbon resin, and hydrogenated product thereof. Amongthem, norbornene resin has favorable transparency and molding propertyand can be suitably used.

The norbornene resins include a ring-opening polymer of monomer having anorbornene structure or a ring-opening copolymer of monomer having anorbornene structure and another monomer, or hydrogenated productthereof, an addition polymer of monomer having a norbornene structure oran addition copolymer of monomer having a norbornene structure andanother monomer, or hydrogenated product thereof and the like.

The transparent resin used in the present invention has a glasstransition temperature of preferably 80° C. or more, and more preferably100 to 250° C. Also, an absolute value of photoelastic coefficient ofthe transparent resin is preferably 10×10⁻¹² Pa⁻¹ or less, morepreferably 7×10⁻¹² Pa⁻¹ or less, and particularly preferably 4×10⁻¹²Pa⁻¹ or less. The photoelastic coefficient C is a value represented byC=Δn/σ when birefringence is Δn and a stress is σ. Using of thetransparent resin with the photoelastic coefficient in this range canresult in restricting a phenomenon of a change in a hue at an edgeportion of a display screen in a liquid crystal display device when aobliquely stretched film obtained by the method of the present inventionis applied to the liquid crystal display device.

The transparent resin used in the present invention may be mixed withcompounding agent such as coloring agent such as pigment and dye,fluorescent brightening agent, dispersing agent, heat stabilizer, lightstabilizer, ultraviolet absorber, antistatic agent, antioxidant,lubricant, solvent and the like as appropriate.

The raw material film used in the present invention may be asingle-layer film or multi-layer film. Also, the raw material film ispreferably supplied as a winding body that the raw material film iswound around a winding core.

In the producing method in the present invention, first, theabove-mentioned long raw material film is longitudinally stretched. Thelongitudinal stretching is to stretch the film in a direction inparallel with the longitudinal direction of the raw material film. Amethod of longitudinal stretching is not particularly limited, forexample, a method of stretching by a difference in rotating speedsbetween rolls can be mentioned.

A longitudinally stretched film having the orientation axis in thedirection substantially parallel with the longitudinal direction and theaverage retardation Re₁ can be obtained by the longitudinal stretching.The average orientation angle of the orientation axis of thelongitudinally stretched film is usually more than 85° and no more than90°, and preferably more than 89° and no more than 90° with respect tothe film width direction. The longitudinally stretched film ispreferably wound around a winding core to have a winding body to besupplied to the subsequent processes. Stretching conditions such as astretching temperature T₁, stretching magnification R₁, speed V₁ and thelike in the longitudinal stretching are not particularly limited, butthe stretching temperature T₁ is preferably Tg−20° C. or more and Tg+30°C. or less, the stretching magnification R₁ is preferably 1.01 times ormore and 10.0 times or less, and the speed V₁ is preferably 1 m/min ormore and 200 m/min or less. Tg is a glass transition temperature of thetransparent resin constituting the raw material film. In thisspecification, angles are noted by inferior angles (smaller angles).

Next, this longitudinally stretched film is stretched in a directionwith the angle θe oblique to the width direction. A condition ofstretching at the oblique angle θe is a condition by which a longobliquely stretched film sample with the average retardation Re₂ isobtained from the long raw material film. Here, Re₂ is smaller than Re₁,preferably smaller than 0.5 times of Re₁. A obliquely stretched filmhaving an orientation axis in a direction larger than the angle θe andlargely different from the angle θe, small retardation (Re) fluctuationor small thickness fluctuation in the plane direction, and theorientation axis uniformly oriented can be obtained by the stretchingunder such conditions.

Therefore, with the method of the present invention, the angle θe can bemade smaller than the angle θs. By making the angle θe smaller, Refluctuation, orientation angle fluctuation and thickness fluctuation ofthe obliquely stretched film obtained by the method of the presentinvention can be further reduced over the wide width of 1300 mm or more.

In order to make Re₂ smaller than Re₁, the stretching temperature of thestep of stretching in a direction of the angle θe is set higher than thestretching temperature of the step of longitudinal stretching; thestretching magnification of the step of stretching in a direction of theangle θe is set smaller than the stretching magnification of the step oflongitudinal stretching; the stretching speed and stretching means(uniaxial stretching or biaxial stretching) are adjusted; or theseconditions are combined.

The stretching magnification of the longitudinal stretching is acquiredfrom the amount of length change in the longitudinal direction, whilethe stretching magnification of the oblique stretching is acquired fromthe amount of length change in the width direction.

The angle θe is preferably 5° or more and 45° or less, more preferably5° or more and 30° or less, and further preferably 5° or more and 15° orless. The angle θe is an angle by which a film having an orientationaxis in a direction of an angle Φ with respect to the width directioncan be obtained from a long raw material film. The angle Φ is preferably5° or more and 45° or less, more preferably 5° or more and 30° or less,further preferably 5° or more and 15° or less. If the angle θe or theangle Φ is in this range, fluctuations in retardation (Re) in thein-plane direction, orientation angle and thickness of the obliquelystretched film obtained by the method of the present invention can befurther reduced over the wide width of 1300 mm or more.

In order to have this obliquely stretching step specifically understood,explanation will be made referring to figures showing an example of theobliquely stretching step.

FIG. 1 is a conceptual diagram illustrating an example of a tenterstretching machine that can suitably put the producing method of thepresent invention into practice. FIG. 2 is a diagram illustratinggrasping means of a rail portion in the stretching machine in FIG. 1.FIG. 3 is a diagram for explaining rail arrangement in the stretchingmachine in FIG. 1. The angle θe is a smaller angle (inferior angle)formed by a line connecting stretch end points E1 and E2 and the filmwidth direction in the fixing zone. A definition of the stretch endpoint will be described later.

The tenter stretching machine shown in FIG. 1 comprises a wind-off roll(winding body of longitudinally stretched film) 21, a wind-up roll 22, atemperature-controlled room 10 composed of a preheating zone A, astretching zone B and a fixing zone C, a rail 11 on which the graspingmeans travels to transport the film and the grasping means 12 (thegrasping means is not shown in FIGS. 1 and 3).

The grasping means 12 grasps the both side ends of the longitudinallystretched film wound off the wind-off roll 21, the longitudinallystretched film is guided into the temperature-controlled room composedof the preheating zone A, the stretching zone B and the fixing zone C tobe obliquely stretched. And the obliquely stretched film is releasedbefore the wind-up roll 22. The obliquely stretched film released fromthe grasping means is taken up by the wind-up roll 22. The pair of rightand left rails 11 have a continuous endless track without a terminal andreturns the grasping means having traveled as above from an outlet sideto an inlet side of the temperature-controlled room.

A longitudinally stretched film 1 is stretched by a tension from thegrasping means while passing through the temperature-controlled roomcomposed of the preheating zone, the stretching zone and the fixingzone.

In the preheating zone, the stretching zone and the fixing zone, apredetermined temperature can be set respectively and the temperature isusually kept constant in each zone. The temperature of each zone can beselected as appropriate, but with respect to the glass transitiontemperature Tg (° C.) of the transparent resin composing the film, thepreheating zone has Tg to Tg+30 (° C.), the stretching zone has Tg toTg+20 (° C.), and the fixing zone has Tg to Tg+15 (° C.).

In the present invention, a temperature difference may be provided inthe width direction in the stretching zone for controlling thicknessfluctuation in the width direction. Particularly in the presentinvention, the temperature close to the grasping means is preferably sethigher than that of the film center portion. In order to provide thetemperature difference in the width direction of the stretching zone,employed can be a known method such as a method of adjusting an openingdegree of a nozzle for feeding hot air into the temperature-controlledroom so that the degree is different depending on the width direction ora method of controlling heaters aligned in the width direction forheating. The lengths of the preheating zone, the stretching zone, andthe fixing zone can be selected as appropriate, and with respect to thelength of the stretching zone, the length of the preheating zone isusually 100 to 150% and the length of the fixing zone is usually 50 to100%.

The grasping means 12 travels on the rail 11 capable of deformation ofarrangement, for example. The rail 11 is arranged so that the film isstretched with the desired angle θe. In FIG. 1, the rail arrangement isset so that the film running direction is made as will be describedlater. In the present invention, the film running direction is adirection of a tangential line connecting mid points in the film widthdirection from the wind-off roll to the wind-up roll.

At a boundary 13 between the preheating zone and the stretching zone anda boundary 14 between the stretching zone and the fixing zone, apartition plate having a slit though which the film can pass isinstalled. The boundary between the preheating zone and the stretchingzone and the boundary between the stretching zone and the fixing zone,that is, the partition plates are preferably at right angles to a filmrunning direction 49 in the fixing zone.

The preheating zone A is a zone for transporting the film while warmingthe film without substantially changing the film length in a directionperpendicular to a film running direction 47 in the preheating zone. Thefilm running direction in the preheating zone is a direction in parallelwith the direction where the film is wound off the wind-off roll and itusually is perpendicular to the rotating shaft of the wind-off roll.

The stretching zone B is a zone for transporting the film whileincreasing the film length in a direction perpendicular to the filmrunning direction in the stretching zone. The film running direction inthe stretching zone is, in the rail arrangement where the stretchingzone is widened at a given angle without changing its inclination as inFIG. 1, a direction of a straight line connecting a mid point of thefilm at the boundary between the preheating zone and the stretching zoneto a mid point of the film in the boundary between the stretching zoneand the fixing zone.

The film running direction in the stretching zone matches the filmrunning direction 47 in the preheating zone in FIG. 1, but it may be outof line. In the case of out of line, it is preferable to be out of linein the direction to upward from the film running direction in thepreheating zone in FIG. 1.

The fixing zone C is a zone for transporting the film while cooling thefilm without substantially changing the film length in a directionperpendicular to the film running direction 49 in the fixing zone. Thefilm running direction 49 in the fixing zone is a direction parallelwith the direction in which the film is taken up by the wind-up roll andusually is perpendicular to a rotating shaft of the wind-up roll.

In the producing method in the present invention, film faces in thepreheating zone, stretching zone and fixing zone are preferablysubstantially parallel with each other. That is, the film wound off thewind-off roll preferably passes the preheating zone, stretching zone andfixing zone without being twisted but staying flat and is taken up bythe wind-up roll.

In the producing method in the present invention, c) traveling speeds ofthe grasping means are substantially equal at both side ends of thefilm. In FIG. 2, a broken line is shown between the opposing pair ofgrasping means. Stretching starts in the neighborhood of the stretchingstart point and ends in the neighborhood of the stretching end point.The stretching start point is a point where an interval between the pairof grasping means begins to be widened, and it is S1 and S2 in FIG. 2.The stretching endpoint is a point where the interval between the pairof grasping means begins to be constant, and it is E1 and E2 in FIG. 2.

The traveling speed of the grasping means can be selected as appropriatebut it is usually 10 to 100 m/minute. A difference in traveling speedsof the right and left pair of grasping means is usually 1% or less,preferably 0.5% or less, more preferably 0.1% or less of the travelingspeed.

In the method for producing an obliquely stretched film in the presentinvention, as shown in FIG. 3, c) the film running direction 49 in thefixing zone is inclined by an angle θ1 from the film running direction47 in the preheating zone. The rail is bent downward in FIG. 3, but itmay be bent upward in line symmetry with the line of the film runningdirection of the preheating zone in FIG. 3 as its axis. The angle θ1 ispreferably the angle θe or less, more preferably (θe−4°) or more and θeor less.

In the stretching zone, the film running direction may be straightwithout changing the direction, or may be changed in a stepwise orcontinuously. The spread angle of the rail can be selected asappropriate according to the stretching magnification.

The running direction in the fixing zone is inclined from the runningdirection in the preheating zone by an angle of θ1 as shown in FIG. 3.Thus, the upper grasping means in the figure takes a longer route thanthe lower grasping means. Therefore, in FIG. 2, when the upper graspingmeans reaches the point E1, the corresponding lower grasping means hasreached the point E2. When seen from the film width direction of thefixing zone, E2 advances to a position ahead of E1.

The film passed through the temperature-controlled room as above isreleased from the grasping means before the wind-up roll and taken up bythe wind-up roll. The film obtained as above has an orientation axisinclined by an angle θs of 10° or more and 85° or less, preferably 40°or more and 85° or less, and more preferably 51° or more and 85° orless. Moreover, the thickness and orientation angle are uniform in thewidth direction.

Also, since the tension from the grasping means acts on the filmuniformly in the width direction, birefringence generated by molecularorientation is uniform in the width direction. According to theproducing method in the present invention, a film with the averageretardation Re (=(n_(x)−n_(y))×d; n_(x) and n_(y) are in-plane principalrefractive indexes of the film, d is a thickness of the film) of 100 to300 nm can be easily obtained. Also, by reducing the angle Φ or byincreasing Re₁ and Re₂, a obliquely stretched film having a large Nzcoefficient can be obtained, while by increasing the angle Φ or byreducing Re₁ and Re₂, a obliquely stretched film having a small Nzcoefficient can be obtained. The Nz coefficient is a value representedby (n_(x)−n_(y))/(n_(x)−n_(y)) when the refractive index in the filmthickness direction is n_(z).

The long obliquely stretched film obtained by the producing method inthe present invention is preferably 1300 mm or more, and more preferably1450 mm or more in width.

The average thickness of the obliquely stretched film obtained by theproducing method in the present invention is preferably 30 to 80 μm,more preferably 30 to 60 μm, and particularly preferably 30 to 50 μm,from the viewpoint of mechanical strength.

In the thickness fluctuation in the width direction of the obliquelystretched film obtained by the producing method in the presentinvention, a difference between the maximum thickness and the minimumthickness is usually 3 μm or less, and preferably 2 μm or less. If thethickness fluctuation is in this range, the obliquely stretched filmobtained by the producing method in the present invention can be takenup lengthily.

The obliquely stretched film obtained by the producing method in thepresent invention has Re fluctuation usually within 10 nm, preferablywithin 5 nm, and more preferably within 2 nm. In the present invention,since the Re fluctuation can be kept in the above range, when used in aliquid crystal display device, its display quality can be madefavorable. Here, the Re fluctuation is a difference between the maximumvalue and the minimum value of Re when Re at a light incident angle 0°(state where an incident light beam and a surface of the obliquelystretched film of the present invention bisect at right angles) ismeasured in the width direction of the obliquely stretched film.

The Nz coefficient of the stretched film obtained by the producingmethod in the present invention is preferably 1.5 or more, morepreferably more than 2.0, and particularly preferably 2.1 or more.

An upper limit value of the Nz coefficient is preferably 10, morepreferably 5, and further preferably 3.

In order to have the Nz coefficient of 1.5 or more, Re₁ is set at 150 nmor more and 1000 nm or less and Re₂ at 0.4 times or more and less than 1times of Re₁. Moreover, at this time, θe is preferably 5° or more and30° or less.

In order to have the Nz coefficient in a range exceeding 2.0, Re₁ is setat 250 nm or more and 1000 nm or less and Re₂ at 0.5 times or more andless than 1 times of Re₁. Moreover, at this time, θe is preferably 5° ormore and 20° or less.

In order to have the Nz coefficient of 2.1 or more, Re₁ is set at 250 nmor more and 1000 nm or less and Re₂ at 0.7 times or more and less than 1times of Re₁. Moreover, at this time, θe is preferably 5° or more and20° or less.

A content of a residual volatile component in the obliquely stretchedfilm obtained by the producing method in the present invention is notparticularly limited but it is preferably 0.1% by weight or less, morepreferably 0.05% by weight or less, and further preferably 0.02% byweight or less. If the content of the residual volatile componentexceeds 0.1% by weight, there is a fear that its optical characteristicswill change over time. By keeping the content of the volatile componentin the above range, dimensional stability is improved, temporal changein average retardation (Re) in the in-plane or average retardation (Rth)in the thickness direction can be reduced, and moreover, deteriorationof a polarizing plate or liquid crystal display device having aobliquely stretched film of the present invention can be restricted,which can keep display of the display device stable and favorable for along time. The residual volatile component is a substance with amolecular weight of 200 or less which is contained in a small amount inthe film, and for example, is a residual monomer, solvent or the like.The content of the residual volatile component can be quantified byanalysis of the film using gas chromatography as a total of substanceswith the molecular weight of 200 or less contained in the film.

A saturated water absorption of the obliquely stretched film obtained bythe producing method in the present invention is preferably 0.03% byweight or less, more preferably 0.02% by weight or less, andparticularly preferably 0.01% by weight or less. By having the saturatedwater absorption in the above range, temporal change in averageretardation (Re) in the in-plane and average retardation (Rth) in thethickness direction can be reduced, and moreover, deterioration of apolarizing plate or liquid crystal display device having a obliquelystretched film obtained by the producing method in the present inventioncan be restricted, which can keep display of the display device stableand favorable for a long time.

The saturated water absorption is a value expressed in percentage of anincreased mass obtained by dipping a test film sample in water at agiven temperature for a given time to a test sample mass before dipping.Usually, measurement is made by dipping in water with a temperature of23° C. for 24 hours. The saturated water absorption in the obliquelystretched film obtained by the producing method in the present inventioncan be adjusted to the above value by reducing an amount of polar groupin a thermoplastic resin, for example, and a resin without a polar groupis preferable.

The long obliquely stretched film obtained by the producing method inthe present invention can be laminated with a long polarizer to obtain along laminate film.

The polarizer used in the present invention transmits one of two linearpolarized lights at right angles to one another. For example, apolarizer obtained by having iodine or dichroic substances such asdichroic dye adsorbed to a hydrophilic polymer film such as a polyvinylalcohol film and ethylene-vinyl acetate partially saponified film andthe like and then, uniaxially stretched, or the one obtained byuniaxially stretching the hydrophilic polymer film and having dichroicsubstance absorbed or a polyene oriented film such as dehydratedpolyvinyl alcohol or dehydrochlorinated polyvinyl chloride and the likecan be mentioned. In addition, reflective polarizers such as gridpolarizer and anisotropic multi-layer film can be mentioned. Thethickness of the polarizer is usually 5 to 80 μm.

The obliquely stretched film obtained by the producing method in thepresent invention may be laminated either on both faces or one face of apolarizer, and the number of laminating layers is not particularlylimited and two or more layers may be laminated.

If the obliquely stretched film is laminated only on one face of thepolarizer, a protective film with a purpose of protecting the polarizermay be laminated on the other face through an appropriate adhesivelayer.

As the protective film, an appropriate transparent film may be used.Among them, a film having resin with transparency, excellent mechanicalstrength, thermal stability, water shielding performance and the like ispreferably used. Examples of such a resin include acetate polymer suchas triacetylcellulose, alicyclic olefin polymer, polyolefin polymer,polycarbonate polymer, polyester polymer such as polyethyleneterephthalate, polyvinyl chloride polymer, polystyrene polymer,polyacrylonitrile polymer, poly sulphone polymer, polyethersulphonepolymer, polyamide polymer, polyimide polymer, acrylic polymer and thelike.

A suitable method for producing a long laminate film in the presentinvention is a method comprising close contact between a obliquelystretched film and a polarizer while winding them off a obliquelystretched film winding body and a polarizer winding body, respectively,at the same time. An adhesive may be interposed in a close-contact facebetween the obliquely stretched film and the polarizer. As a method ofbringing the obliquely stretched film and the polarizer into closecontact, a method of inserting the obliquely stretched film and thepolarizer together into a nip of two rolls aligned in parallel forpressure sandwiching.

A long obliquely stretched film or long laminate film in the presentinvention is used as a retarder or a polarizing plate by cutting it intoa desired size according to its use form. In this case, it is preferableto cut out along a direction perpendicular or parallel to thelongitudinal direction of the long film.

A liquid crystal display device in the present invention is composed ofa retarder or a polarizing plate cut out of the long obliquely stretchedfilm or the long laminate film. An example of the liquid crystal displaydevice of the present invention includes a device constituted by aliquid crystal panel whose polarizing transmission axis can be changedby adjustment of an electric voltage and the polarizing plate of thepresent invention arranged so as to sandwich the panel. Also, theretarder of the present invention is used in a liquid crystal displaydevice for optical compensation and polarizing light conversion and thelike. The liquid crystal display device is usually provided with abacklight device for a transmission type liquid crystal display deviceor a reflecting plate for reflective type liquid crystal display deviceon the back side of a display face in order to feed light into theliquid crystal panel. A light source used for the back light includescold cathode-ray tube, mercury plane lamp, light-emitting diode, EL andthe like. As the liquid crystal display device of the present invention,a reflective type liquid crystal display device composed of a reflectivetype liquid crystal panel is preferable. The liquid crystal panel is notparticularly limited by its display mode. For example, TN (TwistedNematic) mode, STN (Super Twisted Nematic) mode, HAN (Hybrid AlignmentNematic) mode, VA (Vertical Alignment) mode, MVA (Multi-domain VerticalAlignment) mode, IPS (In Plane Switching) mode and the like can becited. In the liquid crystal display device of the present invention, anappropriate part such as prism array sheet, lens array sheet, diffusingsheet, light diffusing plate, light guide plate, brightness improvingfilm and the like may be arranged in one or two layers or more at anappropriate position in addition to the above.

The obliquely stretched film obtained by the producing method in thepresent invention can be applied to an organic EL display device, plasmadisplay device, FED (Field Emission Display) device, and SED(Surface-conduction Electron-emitter Display) device other than theliquid crystal display device.

EXAMPLES

Evaluation is made by the following method in the examples.

(Average Thickness, Thickness Fluctuation)

A thickness was measured with a 5-cm interval in the film widthdirection using a snap gauge (produced by Mitutoyo corporation,ID-C112BS) and an average value was acquired. The thickness fluctuationwas set as a difference between the maximum value and the minimum valuein the thickness.

(Average Orientation Angle, Orientation Angle Fluctuation)

A retardation axis in a plane was measured with a 5-cm interval in thefilm width direction using a polarizing microscope (produced by OlympusCorporation, BX51) and an average value of an angle (orientation angle)formed by a direction of the retardation axis and the width directionwas acquired. The orientation angle fluctuation was set as a differencebetween the maximum value and the minimum value of the orientationangle.

(Average RE, Average Nz Coefficient)

Re and Nz coefficient were measured with a 5-cm interval in the filmwidth direction using a phase difference detector (produced by OjiScientific Instruments, KOBRA21-ADH) according to formulas (1) and (2)and an average value was acquired. The Re fluctuation was set as adifference between the maximum value and the minimum value of Re.Re=(n _(x) −n _(y))·d  (1)Nz=(n _(x) −n _(z))/(n _(x) −n _(y))  (2)

-   -   three-dimensional refractive indexes, n_(x), n_(y), n_(z),        thickness d

Example 1

A pellet of norbornene resin (by Zeon corporation, ZEONOR1420) wasmolded by a T-die type film extrusion molding machine so as to have along unstretched film (A) with a width of 1000 mm and a thickness of 130μm. The unstretched film (A) was taken up by a roll.

Then, the unstretched film (A) was wound off the roll, supplied to alongitudinal stretching machine, and longitudinally stretched under astretching condition shown in Table 1 so as to have a longitudinallystretched film (B) with characteristics shown in Table 2. Thelongitudinally stretched film (B) was taken up by another roll.

Moreover, the longitudinally stretched film (B) was wound off the roll,supplied to a tenter stretching machine shown in FIGS. 1 to 3, stretchedunder the stretching condition shown in Table 1 in a direction obliquelyat 10° to the width direction of the longitudinally stretched film (B)so as to have a obliquely stretched film (C), and the film (C) wasfurther taken up by an added roll so as to have a obliquely stretchedfilm winding body. The film (C) had an orientation axis of an averageorientation angle of 75° with respect to the width direction. Theproperties of the film (C) are shown in Table 2.

TABLE 1 Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 1 Ex. 2 Ex. 3 Ex.4 Film (A) Average 2 2 2 2 2 2 2 Re [nm] Longitudinal Stretching 142 142142 — — 142 142 stretching Temp. [° C.] Stretching 1.8 1.8 1.8 — — 1.81.8 magnification [times] Take-up speed 20 20 20 — — 20 20 [m/min.]Stretching 90 90 90 — — 90 90 direction [°] Re₁ [nm] 273 273 273 2 2 273273 Tenter Re₂ [nm] 85 238 55 200 160 382 365 stretching θ1 [°] 7 32 1675 70 7 19 Stretching 148 142 145 145 155 145 143 temp. [° C.]Stretching 1.5 1.8 1.3 1.6 1.8 2.5 2 magnification [times] Take-up speed20 18 15 16 17 20 18 [m/min.] θe [°] 10 35 20 75 70 10 23 L [mm]* 2761315 494 6231 5161 460 877 Width after 1565 1878 1357 1670 1878 26092086 stretching [mm] Stretched film (C) (D) (E) (F) (G) (H) (I) *L is adistance between a stretching end point E2 − a fixing zone start pointin FIG. 2

TABLE 2 Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 1 Ex. 2 Ex. 3 Ex.4 Film (A) (B) (C) (D) (E) (F) (G) (H) (H) Width [mm] 1400 1043 15651878 1357 1670 1878 2609 2086 Average 130 97 65 54 75 81 72 39 50thickness [μm] Thickness 1 1 2 2 2 10 10 2 3 fluctuation [μm] Average 2273 137 68 185 200 160 182 158 Re [nm] Re 1 3 1 3 2 20 18 3 4fluctuation [nm] Average — 1.0 2.2 1.4 1.6 1.2 1.2 1.8 1.3 Nz [—]Average 0 90 75 45 60 72 68 8 20 orientation angle θs [°] Orientation 11 1 2 1 8 7 1 5 angle fluctuation [°] Presence of none none none nonenone yes yes none none wrinkle

Example 2

The long longitudinally stretched film (B) obtained in Example 1 waswound off the roll, supplied to a tenter stretching machine, stretchedin a direction obliquely at 10° with respect to the width direction ofthe longitudinally stretched film (B) under the stretching conditionshown in Table 1 so as to obtain a obliquely stretched film (D). And thefilm (D) was taken up by an added roll so as to have a obliquelystretched film winding body. The film (D) had an orientation axis of anaverage orientation angle of 45° with respect to the width direction andhad properties shown in Table 2.

Example 3

The long longitudinally stretched film (B) obtained in Example 1 waswound off the roll, supplied to a tenter stretching machine, stretchedin a direction obliquely at 15° with respect to the width direction ofthe longitudinally stretched film (B) under the stretching conditionshown in Table 1 so as to obtain a obliquely stretched film (E).Moreover the film (E) was taken up by an added roll so as to have aobliquely stretched film winding body. The film (E) had an orientationaxis of an average orientation angle of 75° with respect to the widthdirection and had properties shown in Table 2.

Comparative Example 1

According to the method described in Japanese Patent Laid-Open No.2-113920, the long unstretched film (A) obtained in Example 1 wassupplied to the tenter stretching machine set to angle, temperature andstretching magnification shown in Table 1 so as to have a obliquelystretched film (F). The film (F) had wrinkles left on the entire faceand can not be taken up by a added roll. The properties of the stretchedfilm (F) are shown in Table 2.

Comparative Example 2

The long unstretched film (A) obtained in Example 1 was supplied to thetenter stretching machine set to angle, temperature and stretchingmagnification shown in Table 1 so as to have a obliquely stretched film(G). The film (G) had wrinkles left on the entire face and can not betaken up by an added roll. The properties of the stretched film (G) areshown in Table 2.

Comparative Example 3

The long longitudinally stretched film (B) obtained in Example 1 waswound off the roll, supplied to the tenter stretching machine, stretchedin a direction obliquely at 10° with respect to the width direction ofthe longitudinally stretched film (B) under the stretching condition inTable 1 so as to have a obliquely stretched film (H). The film (H) wasfurther taken up by a added roll so as to have a obliquely stretchedfilm winding body. An average orientation angle of the film (H) withrespect to the width direction was 8°. The other properties are shown inTable 2.

Comparative Example 4

The long longitudinally stretched film (B) obtained in Example 1 waswound off the roll, supplied to the tenter stretching machine, stretchedin a direction obliquely at 23° with respect to the width direction ofthe longitudinally stretched film (B) under the stretching condition inTable 1 so as to have a obliquely stretched film (I). The film (I) wasfurther take up by a added roll so as to have a obliquely stretched filmwinding body. An average orientation angle of the film (I) with respectto the width direction was 20°, but the result was a largeorientation-angle fluctuation. The other properties are shown in Table2.

From the results of Comparative Examples 1 and 2, it is known that evenif a obliquely stretched film with an average orientation angle of 45°or more to the width direction is to be obtained, wrinkles are generatedand the film can not be used as an optical film. Also, from the resultsof Comparative Examples 1 to 4, it is known that if Re₂ is at Re₁ ormore, an orientation axis emerges in a direction substantially equal tothe angle θe. At this time, it is known that if the orientation anglebecomes large, fluctuation in physical properties of the obtainedstretched film also becomes large (see Comparative Example, 1, 2, and4).

On the other hand, according to the producing method in the presentinvention, it is known that a obliquely stretched film with an averageorientation angle of 45° or more with respect to the width direction(Examples 1 to 3) can be manufactured without generating a wrinkle.Also, even if the stretching angle θe in the obliquely stretchingprocess is small, an orientation angle of the obtained stretched filmcan be increased, and an orientation axis of the stretched film can begenerated with accuracy in a direction between the stretching directionin the longitudinal stretching process and the stretching direction inthe obliquely stretching process. Moreover, the obtained stretched filmhas a large width and small fluctuation of thickness or of Re.

1. A method for producing a long obliquely stretched film having anorientation axis in a direction with an angle θs of 10° or more and 85°or less from a width direction, the method comprising: a) a step oflongitudinally stretching a long raw material film to obtain alongitudinally stretched film with an average retardation Re₁; and astep of stretching the longitudinally stretched film in a direction withan angle θe oblique to the width direction, wherein b) said step ofstretching in the direction with the angle Oe is performed under acondition that a long obliquely stretched film sample with the averageretardation Re₂ (Re₂ is smaller than Re₁) is obtained from said long rawmaterial film.
 2. The method for producing a long obliquely stretchedfilm according to claim 1, wherein a stretching temperature of thestretching step in said direction of the angle θe is higher than astretching temperature of the step of longitudinal stretching.
 3. Themethod for producing a long obliquely stretched film according to claim1, wherein a stretching magnification of the stretching step in saiddirection of the angle θe is smaller than a stretching magnification ofthe longitudinal stretching step.
 4. A method for producing a longobliquely stretched film having an orientation axis in a direction withan angle θs of 10° or more and 85° or less from a width direction, themethod comprising: a) a step of winding a longitudinally stretched filmoff a winding body of the longitudinally stretched film with the averageretardation Re₁ obtained by longitudinally stretching a long rawmaterial film; a step of grasping both side ends in the width directionof the longitudinally stretched film by grasping means; a step ofstretching the longitudinally stretched film in the direction of theangle θe oblique to the width direction through a preheating zone, astretching zone and a fixing zone to obtain a obliquely stretched film;a step of releasing the both side ends of the obliquely stretched filmfrom the grasping means; and a step of taking up the obliquely stretchedfilm around a winding core, wherein b) said step of stretching in thedirection of the angle θe is performed under a condition that a longobliquely stretched film sample with the average retardation Re₂ (Re₂ issmaller than Re₁) is obtained from the long raw material film; and c)traveling speeds of said grasping means are substantially equal at theboth side ends of the film.
 5. The method for producing a long obliquelystretched film according to claim 4, wherein a film running direction insaid fixing zone is inclined with an angle of θ1 from the film runningdirection in the preheating zone and the angle θ1 is not more than θe.6. A long obliquely stretched film obtained by the method according toclaim
 1. 7. A long laminate film obtained by laminating the longobliquely stretched film according to claim 6 and a long polarizer withtheir longitudinal directions aligned.
 8. A method for producing aretarder comprising a step of trimming the long obliquely stretched filmaccording to claim 6 to a predetermined size along a directionperpendicular to or parallel with its longitudinal direction.
 9. Amethod for producing a polarizing plate comprising a step of trimmingthe long laminate film according to claim 7 to a predetermined size. 10.A liquid crystal display device comprising the retarder obtained by themethod according to claim
 8. 11. A liquid crystal display devicecomprising a polarizing plate obtained by the method according to claim9.
 12. A liquid crystal display device according to claim 10 comprisinga reflective-type liquid crystal panel.